I saw it yesterday and it's very fun. It captures the spirit of the first 2 seasons really well (that's a polite way of saying that I'm not wild about S3). Even if you didn't watch the show -- and you should have, because it drew on Buffy a lot -- you'll be able to follow the movie. If you did watch it, you'll appreciate the in jokes and the recurring characters. Sadly, no Buffy alums appear in the movie, though maybe Krysten Ritter should get an honorable mention for her father's role in Ted.
Going off on a yet bigger tangent, Aeryl pointed out to me that Game of Thrones has cast Dead Eddie from The Freshman for a key role in S4.